(1) Field of the Invention
This invention relates to an abrasive molding and an abrasive disc provided with at least one abrasive molding, which are used in a process for polishing or chemicomechanically polishing substrate materials, for example, for substrates such as a silicon wafer, an oxide substrate, a chemical compound semiconductor substrate, a glass substrate and a silica glass substrate and a ceramic substrate, and optical materials such as an optical lens and a spectacle lens.
(2) Description of the Related Art
With the advance of industries including an optical industry and an electronic industry, a higher precision and other requirements for processing materials for a magnetic disc, a semiconductor substrate, a single crystal material, an optical material and other substrate materials, are becoming severe. That is, there is an increasing demand for obtaining higher smoothness and flatness by polishing the material surface in the finishing process thereof.
A loose abrasive machining has been widely employed in the conventional polishing process, wherein a substrate material is polished with a polishing pad made of nonwoven fabric or suede while a polishing liquid containing a loose abrasive grain is continuously applied onto the polishing surface. The loose abrasive grain is composed of, for example, diamond, aluminum oxide, silicon oxide, cerium oxide, zirconium oxide, iron oxide, titanium oxide, manganese oxide, silicon carbide or zirconium silicate.
The conventional polishing process using a loose abrasive grain has a problem such that a polishing pad used has a very low modulus and thus the substrate material is not uniformly abraded over the entire surface to be polished, i.e., the corner portions of the material surface are excessively abraded upon polishing.
If a polishing pad is used together with a polishing liquid containing no loose abrasive grain, such as water having an adjusted pH value, the polishing power is too weak to complete the polishing within a reasonably short time. To cope with this problem, a salient amount of a loose abrasive grain must be incorporated in a polishing liquid, but, the incorporation of a salient amount of a loose abrasive grain leads to production of a salient amount of a waste polishing liquid containing a loose abrasive grain. Therefore, the efficiency of polishing, the equipment for waste disposal and the environmental pollution with the waste polishing liquid must be considered.
To solve the above-mentioned problems, a proposal has been made in Japanese Unexamined Patent Publication (hereinafter abbreviated to xe2x80x9cJP-Axe2x80x9d) No. H4-256581 wherein a synthetic abrasive stone comprising abrasive grain particles bonded with a synthetic resin binder is used. It is described in this patent publication that the problem of non-uniform abrading can be mitigated or avoided.
Further, vitrified grinding stone comprising abrasive grain particles bonded with an inorganic binder, and a metal bonded grinding stone comprising abrasive grain particles bonded with a metal binder have been proposed. It is said that the problem of non-uniform abrading can be mitigated or avoided by using these inorganic substance-bonded grinding stone.
However, the use of these binders causes another problem such that the abrasive stone tends to be clogged with the binder, leading to reduction of polishing performance and efficiency. Further, it is very difficult to uniformly disperse fine abrasive grain particles In an abrasive stone bonded with the synthetic resin binder or the metal or other inorganic substance binder, in the manufacturing process, and surface defects such as worn marks are liable to be caused in a manner similar to the case where a large abrasive grain Is used. If the amount of fine abrasive grain particles is reduced to enhance uniform dispersion of the grain particles, the rate of polishing, and the polishing performance and efficiency are undesirably reduced. Further, the use of binders such as synthetic resins, metal, and inorganic binders such as glass materials containing an alkali metal and other impurities, occasionally causes contamination of a polished material with impurities from the binders during the polishing process depending upon the abrading conditions.
An abrasive molding predominantly comprised of an abrasive silica grain is described in JP-A H10-264015. The following findings are described In this patent publication.
(1) The abrasive molding has a modulus higher than that of a polishing pad, and thus, excessive abrasion of the corner portions of the material surface occurring upon polishing can be minimized, and the substrate material can be uniformly abraded over the entire surface to be polished.
(2) The abrasive molding has a rough surface composed of silica particles, among which a multiplicity of fine pores are formed, and therefore, a problem such that an abrasive molding tends to be clogged during polishing can be minimized or avoided
(3) The abrasive molding does not contain a synthetic resin or other binder, and hence the abrasive molding exhibits high thermal resistance and chemical resistance in the polishing process. Therefore, a high polishing efficiency can be obtained by using an appropriate polishing liquid in a temperature range up to approximately the boiling point.
(4) The abrasive molding is composed of silica particles used as abrasive grain, and the molding does not contain a binder, and thus, the abrasive molding does not cause contamination of a polished material.
(5) The smooth polished surface and the rate of polishing achieved with the abrasive molding are of the same level as or higher level than those of the conventional polishing processes using a polishing pad. The smooth finish and the rate of polishing are not decreased with a lapse of polishing time.
(6) The molding abrasive has a rough surface composed of silica particles. The hard and fine abrasive surface of the silica particles are brought into direct contact with a material to be polished, and hence, a polishing liquid which does not contain loose abrasive grains can be used for polishing with the abrasive molding.
(7) Even if an abrasive loose grain is used in combination with the abrasive molding, a high rate of polishing can be achieved with a polishing liquid containing the abrasive loose grain at a low concentration, as compared with the conventional polishing process using a polishing pad.
A grinding stone consisting of sintered body of inorganic abrasive grains is described in JP-A H10-337669. It is taught in this patent publication that good results similar to those obtained by the abrasive molding of JP-A H10-264015, can be achieved by suitably selecting the material and particle size of abrasive grains, and the porosity and water absorption of the grinding stone. However, the polished surface of silicon wafer as an example of the material to be polished exhibits a surface roughness of approximately 3 nm as expressed in terms of center line mean surface roughness. The rate of polishing is not referred to in this patent publication.
In the above-stated JP-A H10-264015, the surface roughness of a polished silicon wafer is expressed in terms of those values as measured by using a universal surface tester SE-3C available from Kosaka kenkyusho K. K. But, we found some difficulty in accurately measuring the surface roughness of a polished surface having a very low roughness by the same surface tester. Thus, we repeated the measurement of surface roughness of the polished surface obtained by the abrasive molding described in JP-A H10-264015, by using an atomic force microscope (AFM; xe2x80x9cSPI3600xe2x80x9d available from SII Co.), and found that the polished surface has a center line mean surface roughness of 0.6 nm to 1 nm, namely, the surface roughness is better than that of the polished surface obtained by the grinding stone described in JP-A H10-337669.
The abrasive molding composed of silica particles used as abrasive grain, described in JP-A H10-264015, is suitable for polishing machining process or chemicomechanical polishing process (hereinafter abbreviated to xe2x80x9cCMP processxe2x80x9d) for substrate materials such as a silicon wafer, an oxide substrate, a chemical compound semiconductor substrate, a glass substrate, a crystalline silica glass substrate and a ceramic substrate, and optical materials. But, the polishing performance attained varies depending upon the material to be polished, and thus, full consideration must be given for selection of material of abrasive grain used and particle size thereof, depending upon the particular material to be polished.
In view of the foregoing state of the prior art, an abrasive molding has been eagerly desired, which is capable of polishing a material to be polished, at a high polishing rate by using a polishing liquid containing no abrasive grain, to give a smooth polished surface having a high surface precision, and which is characterized by an enhanced polishing efficiency and a reduced polishing cost.
A primary object of the present invention is to provide an abrasive molding, which is suitable for polishing machining process or CMP process for substrate materials such as a semiconductor substrate, an oxide single crystal substrate, a glass substrate, a crystalline silica glass substrate and a ceramic substrate, and for optical materials for which a high precision machining is required, and further to provide a polishing disc provided with at least one of the abrasive molding.
More specifically, a primary object of the present invention is to provide an abrasive molding and a polishing disc provided with at least one abrasive molding; which abrasive molding is capable of polishing a material to be polished with a high efficiency by using a polishing liquid containing no loose abrasive grains or containing a minor amount of loose abrasive grains, and thus, the polishing cost is reduced and the problem of waste polishing liquid containing loose abrasive grains is mitigated; and is capable of polishing the material with a higher efficiency to give a smooth polished surface of the same level as or higher level than those of the conventional polishing processes using a polishing pad.
In accordance with the present invention, there is provided an abrasive molding for polishing a material to be polished by using a polishing liquid containing no loose abrasive grain, said molding consisting essentially of inorganic particles having an average particle diameter in the range of 0.005 xcexcm to 0.3 xcexcm, and said molding having a relative density in the range of 45% to 90%, provided that pores having a diameter of at least 0.5 xcexcm are excluded from the molding.
In accordance with the present invention, there is further provided an abrasive disc for polishing a material to be polished by using a polishing liquid containing no loose abrasive grain, said abrasive disc comprising at least one abrasive molding fixed to a supporting auxiliary; said abrasive molding comprising inorganic particles having an average particle diameter in the range of 0.005 xcexcm to 0.3 xcexcm, and said molding having a relative density in the range of 45% to 90%, provided that pores having a diameter of at least 0.5 xcexcm are excluded from the molding.